The present invention relates to a dry etching method and, more particularly, to improved anisotropic dry etching on a lamination of a silicon base film, a tungsten film, and a tungsten alloy film, which dry etching is adopted mainly in a process of manufacturing a high-performance semiconductor device having a gate width narrower than 0.18 xcexcm rules.
In recent years, semiconductor devices have remarkably grown in performance, and further speedup and reduction in power consumption have been demanded.
For example, in a fine semiconductor device having a gate width narrower than 0.18 xcexcm rules, for further improvement in performance of transistors, there has been promoted a transition from a conventional N+ single gate structure to a dual gate structure in which a P+ electrode is used for a P channel transistor while an N+ electrode is used for an N channel transistor. On the other hand, for further speedup and reduction in power consumption, reduction in resistance of gate electrodes has been demanded.
Furthermore, in order to secure a margin for alignment between a gate electrode and an adjacent contact region, SAC (Self Align Contact) process has been employed.
As a gate electrode structure that satisfies the above-mentioned demands on the semiconductor device, a poly-metal electrode using tungsten is promising.
A poly-metal electrode is a kind of gate electrode structure, and it is not an ordinary gate electrode comprising only poly-silicon but a gate electrode made by stacking a metal layer such as tungsten on poly-silicon. The poly-metal electrode realizes a lower resistance as compared with the gate electrode comprising poly-silicon alone, whereby speedup and reduced power consumption are achieved. Further, since the poly-metal electrode is consistent with the SAC process, it is suitable for high-density integration.
Hereinafter, a method for fabricating a poly-metal electrode structure will be described with reference to FIGS. 1(a)-1(c).
Initially, there are successively formed on a semiconductor substrate such as a silicon substrate 11, a gate oxide film 12 (e.g., 3 nm thick), a poly-silicon (poly-Si) film 13 (e.g., 100 nm thick), a tungsten nitride (WNx) film 14 (e.g., 10 nm thick), a tungsten (W) film 15 (e.g., 100 nm thick), and a silicon nitride (SiN) film 16 (e.g., 150 nm thick), in this order. Preferably, the gate oxide film 12 is formed by thermal oxidation, and the poly-Si film 13 is formed by CVD (Chemical Vapor Deposition). Further, the WNx, film 14 and the W film 15 are formed by sputtering or CVD, and the SiN film 16 is formed by CVD.
Next, the W film 15, the WNx film 14, and the poly-Si film 13 are subjected to anisotropic etching, using the SiN film 16 and the resist pattern 17 as masks, in an etching apparatus different from that used for the SiN film 16, thereby completing a poly-metal structure gate electrode (refer to FIG. 1(c)).
In this structure, the WNx film 14 functions as a barrier layer that prevents mutual diffusion between the W film 15 and the poly-Si film 13, and the SiN film 16 functions as a stopper layer in the SAC process.
Conventionally, as anisotropic etching of tungsten, low-temperature etching using SF6 gas and high-temperature etching using Cl2 gas have been carried out, as disclosed in the transactions of JSAP (Japan Society of Applied Physics) annual meetings as follows: 1989 Autumn JSAP annual meeting (Tsujimoto et al., No.2, p.464), 1991 Spring JSAP annual meeting (Kato et al. No.2, p.503, 28p-ZC-14), 1992 Autumn JSAP annual meeting (Iida et al., No.2, p.465, 16a-SK-8), and 1992 Autumn JSAP annual meeting (Hayashi et al., No.2, p.465, 16a-SK-9).
By the way, tungsten (W) has conventionally been used as a material for plugs, interconnections (bit lines) in DRAM, or the like. A typical wiring structure comprises a W layer having a thickness of 300xcx9c500 nm, a barrier layer comprising Ti or TiN and placed under the W layer, and a base comprising an oxide film as an inter-layer film.
In this structure, when using SF6 as a main gas, anisotropic dry etching of W proceeds in a reaction as follows;
W+6Fxe2x86x92WF6↑
The reason why SF6 gas is used as an etching gas is because the vapor pressure of WF6 as a reaction product is high (i.e., WF6 is easy to volatile) and a lot of F radicals are generated, whereby the relatively thick W film can be etched at a practical etching rate (400xcx9c600 nm/min).
However, if the etching gas used in the conventional dry etching of W is applied to, as it is, etching of the poly-metal structure shown in FIG. 1, the following drawbacks will occur.
Firstly, since the thickness of the W film 15 (100 nm or less) used in the poly-metal structure is less than that of the W layer (300xcx9c500 nm) used in the bit line wiring or the like, the etching rate by SF6 is too high to control.
Secondly, SF6 cannot perform selective etching of the W film 15 with respect to the underlying poly-Si film 13 and, moreover, the etching rate by SF6 is higher in the poly-Si film 13 than in the W film 15. Therefore, in combination with the first problem, etching does not stop in the poly-Si film 13, and penetrates through the gate oxide film 12. As the result, a transistor cannot be fabricated.
Further, in the etching process to form the poly-metal electrode structure shown in FIG. 1, a vertical etching configuration and a high selectivity with respect to the gate oxide film 12 are demanded. To meet these demands, it is effective to use HBr base gas for etching of the poly-silicon film 13. However, the HBr base gas is improper as a main gas for etching of the W film 15 because the vapor pressure of WBrx is low.
Accordingly, in the etching process to form the poly-metal electrode structure shown in FIG. 1, it is difficult to use the same gas for etching of the W and WNx films 15 and 14 and for etching of the poly-Si film 13 and, therefore, these etching processes should be carried out under different conditions.
For example, when etching of the poly-metal structure is carried out by the conventional method described above, etching of the W and WNx films 15 and 14 should be carried out on a susceptor (a lower electrode serving as a substrate holder in the dry etching apparatus) having a low temperature (about xe2x88x9220xc2x0 C.) when using SF6 gas while it should be carried out on a susceptor having a high temperature (100xc2x0 C. or higher) when Cl2 gas is used. These temperature ranges are necessary to obtain vertical configurations while suppressing side-etching of the W and WNx films. However, if etching of the poly-Si film using HBr base gas is carried out under these temperature ranges, it is difficult to obtain the vertical configuration in either case.
This problem is described in the transaction of 1998 Autumn JSAP annual meeting (Kawakami et al., No.2, p.640, 16p-C-3), and an etching apparatus having plural etching chambers which are adapted to the properties of plural layers to be etched is required for etching to form a poly-metal gate electrode.
Therefore, there are considerable drawbacks in mass production, such as an increase in the initial cost of the etching apparatus, an increase in the failure rate due to the complicated device system, a reduction in the operation rate, and the like.
There is another problem about the selectivity of the underlying poly-Si film while etching the W and WNx films. If the selectivity of the poly-Si film is low in etching of the W and WNx film, etching does not stop in the poly-Si film and penetrates through the gate oxide film.
In order to increase the selectivity of poly-Si, for example, Japanese Published Patent Application No. Hei.9-82686 discloses a method of mixing oxygen into the etching gas to form a surface oxide film on poly-Si.
However, if the oxygen density is higher than 50% as described in this bulletin, when etching of the W and WNx films is completed, a thick oxide film is formed on the underlying poly-Si surface or deposition occurs, whereby etching does not proceed furthermore. In order to remove the thick oxide film or the deposition in the following poly-Si etching, an etching condition having low anti-oxide-film selectivity must be used, and such etching condition leads to penetration through the gate oxide film. Therefore, it is practically difficult to perform, by this method, etching of the poly-metal structure with high controllability.
The present invention is made to solve the above-described problems and has for its object to provide a dry etching method by which a poly-metal gate structure including tungsten can be fabricated at high mass-productivity while keeping the vertical configuration of the structure with high controllability.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will he apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided a dry etching method for etching a structure obtained by successively depositing, on a substrate, a gate insulating film, a silicon base film, a tungsten film or an alloy film containing tungsten, wherein the dry etching includes a first process of dry-etching the tungsten film or the alloy film including tungsten, and a second process of dry-etching the silicon base film, and the first process employs, as an etching gas, a gas mixture obtained by mixing O2 gas into a gas including at least C and F, with the flow ratio of the O2 gas being 10xcx9c50% by volume percentages. According to the dry etching method of this first aspect, highly precise dry etching is achieved. That is, since the CF base gas is used as a main gas, a vertical configuration of W is easily obtained when the temperature of a susceptor is within a range from 0xc2x0 C. to 70xc2x0 C. This is caused by the effect of side-wall protection by C component. That is, since isotropic etching of W by F is suppressed by C component, vertical etching is achieved in the above-mentioned temperature range. Further, addition of O2 increases the selectivity of the W film with respect to the underlying poly-Si film, that is, the poly-Si film becomes resistant to etching, whereby the possibility of penetration through the gate oxide film when etching the W film is prevented. Accordingly, this dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the underlying gate oxide film is prevented.
According to a second aspect of the present invention, in the dry etching method of the first aspect, the first process and the second process are carried out in the same chamber. Also this dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the underlying gate oxide film is prevented. In addition, etching of the poly-metal structure can be carried out in one chamber, resulting in dry etching that is superior in mass-productivity to the conventional methods.
According to a third aspect of the present invention, in the dry etching method of the first aspect, the first process and the second process are carried out at the same susceptor temperature. Also this dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the underlying gate oxide film is prevented In addition, since etching of W or W alloy and etching of poly-Si can be carried out at the same temperature, etching of the poly-metal structure can be carried out in one chamber, resulting in dry etching that is superior in mass-productivity to the conventional methods.
According to a fourth aspect of the present invention, in the dry etching method of the first aspect, N2 gas is further added to the etching gas used in the first process. Addition of the N2 gas has the effect of smoothing the surface of the poly-Si film when etching the W film. This is caused by that N2 or a reaction product including N, which has deposition property, is deposited on concave portions at the poly-Si surface. Accordingly, this dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure having a very thin gate insulating film as a base, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the gate insulating film is prevented.
According to a fifth aspect of the present invention, in the dry etching method of the fourth aspect, the mixing ratio of the N2 gas in the etching gas used in the first process is determined such that the flow ratio of the N2 gas to the gas including at least C and F is 2xcx9c20% by volume percentages. This dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure having a very thin gate insulating film as a base, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the gate insulating film is prevented.
According to a sixth aspect of the present invention, in the dry etching method of the first aspect, one of Cl2, HCl and HBr gases is further added to the etching gas used in the first process. Addition of the Cl2, HCl, or HBr gas has the effect of increasing the etching rate while maintaining anisotropy in etching of the W film. Accordingly, this dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure having a thick metal film, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the underlying gate oxide film is prevented.
According to a seventh aspect of the present invention, in the dry etching method of the sixth aspect, the mixing ratio of the Cl2, HCl or HBr gas in the etching gas used in the first process is determined such that the flow ratio of this gas to the gas including at least C and F is 2xcx9c20% by volume percentages. This dry etching method realizes highly-precise dry etching by which a vertical configuration is obtained in a poly-metal electrode structure having a thick metal film, and the selection ratio of W to poly-Si can be controlled and, moreover, penetration through the underlying gate oxide film is prevented.